It was made for just a few thousand pounds, stars unpaid student actors and was designed to warn youngsters in a corner of south Wales about the perils of texting while driving.

They may not seem the obvious ingredients for international success, but the four-minute film made by Gwent police has become an internet phenomenon, which has now been seen by millions of people and featured on TV news channels in the US.

Though, until now, it has been unheralded in the UK, the film has fuelled debates about road safety and prompted people from India to Canada to contact the Welsh force to describe how affected they have been by the images.

In America the advertising guru Donny Deutsch said it may be one of the most powerful public information films ever made and its disturbing content has been pored over by the likes of the Washington Post, CNN and Time magazine.

The film tells the story of a fictional 17-year-old girl, Cassie Cowan (nickname COW), who is distracted by her mobile for a few seconds while driving with two friends.

COW – a "nice girl from a nice valleys family" – causes a devastating crash which kills her friends and another couple. The impact and its aftermath is portrayed in vivid, harrowing and bloody detail.

One girl's face hits the windscreen with sickening force. A child in the car COW crashes into asks: "When will mummy and daddy wake up?" while a baby strapped into a child seat stares unblinking and may be dead.

Actually, police had intended to commission a different film – on joyriding. But when they spoke to pupils at Tredegar comprehensive, the youngsters told them that texting while driving was a much more important issue for them.

With a budget of £10,000, film-maker Peter Watkins-Hughes was asked to write and direct the film. Local people donated props including the cars that are smashed up and the locations while Watkins-Hughes and his cast gave their time.

A 30-minute version of the film is due to be shown for the first time this autumn but Watkins-Hughes put a four-minute clip of it on YouTube (entitled COW test 001) to show it to a friend. For weeks the clip remained unnoticed by anyone but the friend and a few crew members.

Then suddenly it began to attract hits. It was copied on to other sites, attracted attention around the world and within a couple of weeks became one of the most popular viral videos. Today it was still ninth on one global viral video chart. Clips about Oasis and Jay-Z were at first and second place.

Watkins-Hughes said it felt like being in an Ealing comedy when a small Welsh community had suddenly attracted worldwide attention.

The "weirdest" moment for him was when his explanation, "We've gone for grim reality", was a quote of the day on the Time website.

Watkins-Hughes said he thought it was so powerful because the violence of the crash was shown but the film then "lingered" on the human price – the baby, the child asking about his mother and father. And the screams of COW, who survives the crash.

He said: "It sounds crass but if we can get one person to change their behaviour then it will have been worthwhile."

Jenny Davies, 18, who plays COW, said she was "overwhelmed" by the response. She was at a music festival when she heard the clip was going global – and CNN and CBS were after her for an interview. CBS sent a crew to Wales to interview Davies, who is on a gap year, and others involved in the production.

The film has struck such a chord in the US where the danger of texting while driving is a big issue because it is not illegal in all states. The trend in America has been to try to get the anti-texting message across more gently through humour or playing on the emotions but not showing violence.

However, one survey in the US found that 80% of people who had seen the Gwent film were less likely to text while driving than before.

In the UK, the road safety charity Brake, having been asked to watch the clip by the Guardian, praised the film-makers and said it was important to show the reality of road crashes.

Gwent's chief constable, Mick Giannasi, said he was astonished but encouraged by the way the film had spread.

He said: "The messages contained in the film are as relevant to the people of Tennessee as they are to the residents of Tredegar. The more this film is viewed, the better."

The plan is still to show the film – which can be viewed at gwent.police.uk – in schools in south Wales but it may now also reach thousands of classrooms in the US.

Any money made will be used to set up a trust fund to enable similar films to be made. "We want there to be a COW legacy," said Watkins-Hughes.

"This is a great positive story for this area and the whole place is so proud."